alloca - allocate memory that is automatically freed
void *alloca(size_t size);
The alloca() function allocates size bytes of space in the stack
frame of the caller. This temporary space is automatically freed when the
function that called alloca() returns to its caller.
The alloca() function returns a pointer to the beginning of the allocated
space. If the allocation causes stack overflow, program behavior is undefined.
For an explanation of the terms used in this section, see attributes(7).
This function is not in POSIX.1.
There is evidence that the alloca() function appeared in
32V, PWB, PWB.2, 3BSD, and 4BSD. There is a man page for it in 4.3BSD. Linux
uses the GNU version.
The alloca() function is machine- and compiler-dependent. For certain
applications, its use can improve efficiency compared to the use of
malloc(3) plus free(3). In certain cases, it can also simplify
memory deallocation in applications that use longjmp(3) or
siglongjmp(3). Otherwise, its use is discouraged.
Because the space allocated by alloca() is allocated within
the stack frame, that space is automatically freed if the function return is
jumped over by a call to longjmp(3) or siglongjmp(3).
The space allocated by alloca() is not automatically
deallocated if the pointer that refers to it simply goes out of scope.
Do not attempt to free(3) space allocated by
Normally, gcc(1) translates calls to alloca() with inlined code.
This is not done when either the -ansi, -std=c89,
-std=c99, or the -std=c11 option is given and the header
<alloca.h> is not included. Otherwise, (without an -ansi or
-std=c* option) the glibc version of <stdlib.h> includes
<alloca.h> and that contains the lines:
#define alloca(size) __builtin_alloca (size)
with messy consequences if one has a private version of this
The fact that the code is inlined means that it is impossible to
take the address of this function, or to change its behavior by linking with
a different library.
The inlined code often consists of a single instruction adjusting
the stack pointer, and does not check for stack overflow. Thus, there is no
NULL error return.
There is no error indication if the stack frame cannot be extended. (However,
after a failed allocation, the program is likely to receive a SIGSEGV
signal if it attempts to access the unallocated space.)
On many systems alloca() cannot be used inside the list of
arguments of a function call, because the stack space reserved by
alloca() would appear on the stack in the middle of the space for the
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